Bragg gratings have a number of important applications in optical components, such as lasers, sensors and dispersion-compensation devices. A Bragg grating formed in a photonic waveguide reflects a characteristic wavelength or range of wavelengths centred at a characteristic wavelength, referred to as the Bragg wavelength λB. The Bragg wavelength depends on an effective refractive index of the waveguide neff and a period Λ of refractive index variations in the grating, according to the equation:λB=2neffΛ  (1)The period of the grating Λ may be constant, in which case a narrow range of wavelengths are reflected, or the period may intentionally change throughout the grating and cause reflections over a range of wavelengths. Bragg gratings can been formed in optical fibers and planar waveguides. A planar waveguide, also referred to as a planar lightwave circuit (PLC), is an optical waveguide formed by one or more thin transparent films on a generally-planar substrate. Planar waveguides confine optical power within a region referred to herein as the core. The core has a higher refractive index than surrounding material and is generally configured as either a channel waveguide, or rib waveguide.
The reflectance spectrum of a Bragg grating typically comprises a central reflectance peak located at the Bragg wavelength λB and multiple side lobes which have lower reflectances than the central peak and are located at wavelengths either side of the central peak. The ratio of the magnitude of the largest side lobe to the magnitude of the central peak is referred to as the side-lobe suppression ratio (SLSR), usually expressed in dB. Most, if not all, applications of Bragg gratings require the SLSR to be as low as possible. However, a tailored SLSR can be difficult to achieve, particularly for planar gratings. For some applications, such as external-cavity lasers, it is much more critical to suppress the side lobes located at wavelengths greater than λB (referred to as the “red” side of the central peak) than the side lobes located at wavelengths lower than λB (referred to as the “blue” side of the central peak). This is because the phase conditions for lasing on the blue side tend to be spaced further from the central peak where than on the red side (the magnitudes of side lobes tend to reduce with increased spectral distance from the central peak). The prior art does not disclose a technique for increasing the suppression of side lobes on the red side of the central peak relative to side lobes on the blue side, or vice versa.